Mercury-cathode electrolytic cell



Sept. 27, 1955 R. D. BLUE ET AL .cERcURY-CATHODE ELECTROLYHC CELL 2 Sheets-Sheet l Filed Oct. 4, 195C SN mm NN N y QW @mu mm m wm k www@ O Q O O O O 0\ lalioflwllf Q o Q o o o o o Sept. 27,y 1955 R. D. BLUE ET AL MERCURYCATHO1)E ELECTROLYTIC CELL 2 Sheets-Sheet 2 Filed OGb. 4, 1.950

,mi QJWMN MN S. R 0 4 4 m w NW 5 u. I W QNX lu TU ITJIIIL rll\ W mm... mw mNE Nw 1 1 in .u wm, KWK L SN a f |,P UL.V|\|| \...||D|. m. .o l IJUHMHI.. O No mw" 1| r |lL\ R SII l mb m2 @Gm 11 m N .VZF/V'fl Q Sv b K um .www .TU, [NN .i wmv, H J| Tw.m j u NN w Q mm Nw @dm Q NN Nw mm2. \|\|w\m|m|m \|1\ L! oeoe N G 3 Q D Q O, N mmm r I Mmmm www i 1 1 0 0 0 Q Q 0 Q wwww i o El L Nw mw, N wm, w i QN A wm mm wm mm N m wm A TTORNE YS United States Patent O MERCURY-CATHODE ELECTROLYTIC CELL Robert D. Blue and Marshall P. Neipert, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application October 4, 1950, Serial No. 188,382

1 Claim. (Cl. 204--220) This invention relates to an improved mercury-cathode cell for the electrolysis of alkali metal halide brines.

ln present commercial mercury-cathode electrolytic plants for the manufacture of chlorine and caustic soda, each cell series is almost invariably formed of a number of individually housed unit cells with external electrical connections. Such installations, while adequate in many respects, have a number of disadvantages. The individual housing of the unit cells incurs high initial construction expense, makes extravagant demands for floor space, and calls for high operating labor costs. In addition, there is inordinate heat loss from the cells by radiation, and the presence of external connections adversely affects power eiciency.

It is a principal object of the present invention to provide an improved cell series of the mercury-cathode type which avoids these disadvantages.

This object is attained in a novel cell series of the vertical filter press modified bipolar electrode type. This type of series, as is known, consists of a number of superposed frames each holding one anode and one cathode assembly, the frames together defining a sequence of abutting unit cells in which the cathode of each cell is electrically connected within the cell frame to the anode of the succeeding cell. In the present invention, each unit cell is of the fiowing mercury-cathode type, and special means are provided for maintaining the mercury stream in each unit cell electrically isolated from the streams in the adjacent cells. In a preferred arrangement, the mercury stream is passed in sequence through all the unit cells of the series before being sent to a conventional amalgam decomposer.

The principles of the invention may be explained in detail with reference to the accompanying diagrammatic drawings, in which:

Fig. l is a longitudinal elevation of a preferred ce1! series, in section along the line 1-1 in Fig. 2;

Fig. 2 is a plan of the assembly of Fig. l, with the top cover removed, i. e. along the line 2 2 in Fig. l;

Fig. 3 is a sectional elevation through the assembly, along the line 3-3 in Figs. l and 2;

Fig. 4 is an enlarged sectional elevation of the mercury outlet end of one of the troughs;

Fig. 5 is a detailed plan view, partly in section, of one of the graphite anodes shown generally in Figs. 1-3;

Fig. 6 is a sectional elevation of an anode, along line 6--6 in Fig. 5; and

Fig. 7 is a detailed sectional elevation of one of the devices for breaking the electrical continuity of the owing mercury, taken along the line 7-7 in Fig. 4.

the

Referring particularly to Figs. l to 3, the cell series shown is made up of a number of vertically superposed substantially horizontal elongated framed troughs (three are shown and indicated as 11, 11a, and 11b), together with a cover 12 for the top, The troughs, which carry the mercury stream constituting the cathode and thus function as electrolysis chambers, are all identical but are so stacked that each one is reversed end-for-end with respect to the trough immediately above it. The assembly rests on a support indicated only schematically as 13.

Each trough consists essentially of a rectangular frame 14 of non-conducting impervious material, such as reinforced granite concrete, formed integrally with a bottom partition 15 of the same material. in each trough is a cathode assembly comprising a continuous ilat sheet 16 of steel or other conducting material wetted by mercury, extending the full inside width of the frame and the entire length thereof except near each end. The sheet is em bedded in the partition 15 except for its upper surface.

Welded to the under side of the cathode sheet 16 at spaced intervals, and likewise embedded in the partition 15, are a number of internally threaded pipe caps 17. Into these caps are screwed graphite connector rods 18 which are also embedded snugly in the partition 15 but extend vertically downward through and beyond it. A at horizontal graphite anode plate 19 is secured to the lower end of each rod 18, the fit being by means of a tapered socket in the center of the anode (Figs. 5 and 6). These anode plates are pierced by numerous small holes 20, and each graphite rod 18 is also drilled with a hole 21 from its end to a point above the anode, to allow upward venting of anode gas produced during electrolysis.

In the cell series illustrated, the individual frames 14 rest on stacks of shims 22 (Fig. l), which serve to control, and permit adjustment of, the spacing between adjacent troughs 11, 11a, etc. This spacing is regulated so that each cathode 16 is inclined from the horizontal at a slight angle to provide easy flow of the mercury, e. g. 0.2 degree, and such that the slope of each is opposite to that of the cathode of the next higher trough.

When the troughs are assembled in the manner described, there are formed a number of superposed electrolysis chambers 23, each bounded on the bottom by a cathode 16, on the sides by a frame 14, and on the top by the under side of the partition 15 of the next higher frame. The ends of each chamber 23 are defined by cross-partitions 24 near the ends of each frame 14. Each such partition, near its juncture with the horizontal partition 15, is formed with a horizontal slit 25 which is the full width of the chamber and leads to the adjacent inlet or outlet well for circulating mercury, more fully described later. Each chamber is rendered gas-tight by a tubular rubber gasket 26 which lls the gap between the opposing ilat faces of the two adjacent concrete frames. Fresh brine to be electrolyzed is circulated to each chamber through inlets 27 in the sides of the frame 14 at one end. Spent brine and gaseous anode products are withdrawn together through similar outlet holes 28 at the other end, the elevation of these latter holes above the cathode 16 serving to ix the depth of the electrolyte in the chamber.

In each of the unit cells, it is important that the spacings between the cathode 16 and the individual anodes 19 be the same throughout the length of the chamber 23. Since adjoining frames are at slight but opposite angles to the horizontal, and hence also at an angle to each other, it is necessary that the individual anode support rods 18 be tted in their holders 17 so that the effective lengths thereof increase from the higher to the lower end of the electrolysis chamber (see Fig` l). Compensation for wear of the anodes during prolonged operation of the cell may be made by removing some of the shims 22.

The topmost frame 11 is surmounted by the cover 12, of the same material as the frames, to form the uppermost electrolysis chamber 23. The anodes 19 for the top chamber are supported on graphite rods 18 secured in the cover 12 in a manner entirely analogous to that of the rods of the lower frames. Current to the cell series is supplied from a bus-bar 29 to which the individual anode rods 18 are connected by clamps 30 (Fig. l).

3 Similarly, the graphite rods 18 projecting beneath the lowest frame 11b are connected to a corresponding busbar 31 of opposite polarity by clamps 32.

The flowing mercury forming the active cathode enters each cell 23 through a hard-rubber guide funnel 33 which empties into an inlet well 34 in an extension of the concrete frame 14 at the higher end. This well, which is somewhat deeper than the electrolysis chamber 23, is provided with a submerged horizontal stilling bafle 35. A hard-rubber partition 36 inset cross-wise in the frame 14 projects over the entrance of the inlet slit 25 by which the well 34 communicates with the cell chamber 23 and also dips near the bottom of the well to form a mercury trap. The mercury leaves each cell through a similar outlet well 37. As shown especially in Fig. 4, the slit by which this well connects with the cell chamber 23 is faced at its outlet side with hard-rubber plates 38 set in the frame 14. Here also a hard-rubber partition 39 extends across the well 27 its full width and dips near the bottom to provide a mercury trap. The trap overows into an outlet hole 40 provided with electric current-interrupting means later described, the hole being in registry with the inlet of the next lower frame and also containing the funnel 33 leading to the next lower unit cell. This funnel is cast at its top into the bottom of the frame 14.

Each outlet well 37 and outlet 40 is formed with hand holes 41 (Figs. 2 and 3) cut mainly in the frame 14 of the cell itself, but also partly in the bottom edge of the next higher frame, to allow access for cleaning the mercury In practice, the exposed portions of both the inlet and outlet ends of each cell are provided with removable rubber covered steel covers (not shown) for safety and cleanliness.

The outlet funnel 33 of the lowest cell enters directly the inlet 42 of a horizontal amalgam decomposer 43 of the type already well known, the details of which form no part of this invention. The mercury flows by gravity through the decomposer 43, and leaves by an outlet 44. From there, a pump 45 returns the mercury through a line 46 to a hollow concrete inlet piece 47 supporting the funnel 33 leading to the top cell chamber. Pure water is supplied to the decomposer 43 through an inlet 48, and concentrated efuent and gaseous products leave together through an outlet 49.

An important feature of the cell series is the device by which the body of flowing mercury in each cell chamber 23 is isolated electrically from the mercury in the next lower cell. One such device is mounted in the outlet 40 of each cell frame, and one is placed within the top outlet 40 and is cast into the frame 14 slightly below the f top of the outlet, forming, in effect, a small vessel with a perforated bottom. Directly beneath each row of holes 51 in the plate 50 one of an array of hard-rubber baille rods 52 is xed horizontally across the outlet a short distance below the plate, the rods being of larger diameter than the holes 51 and in a common plane. A second tier of similar baflie rods 53 is fixed across the outlet 40 parallel to but lower than the rods 52, the individual rods 53 being staggered with respect to the higher rods 52.

In operation of the device, mercury overowing the well 37 enters the outlet 40 and spreads over the plate 50. From there, it falls through the holes 51 as a series of continuous streams which impinge on the baille rods 52. The impact breaks each stream into a shower of discrete droplets, which fall downwardly. The drops are further subdivided by hitting the lower rods 53. The entire shower descends into the funnel 33, in which the droplets are collected and coalesce, leaving the bottom outlet of the funnel as a continuous stream. Because the mercury stream is broken into showers of distinct series.

droplets which are separate from one another, electric current cannot traverse the shower. In consequence, the mercury on the upper plate 40 is electrically isolated from the mercury stream leaving the bottom of the funnel 33.

In addition to the apparatus illustrated, the assembled cell series is provided with a conventional piping system, not shown, for supplying brine to be electrolyzed to the individual cell chambers 23 through the inlets 27, and for withdrawing spent brine and gaseous anode products through the outlets 28, separating the gaseous products. and reworking the spent brine.

rl`he operation of the cell series may be explained with reference to its most common use, i. e. the electrolysis of alkali metal halide brine, usually saturated sodium chloride brine, to produce gaseous chlorine and sodium hydroxide solution as final products. A sutiicient quantity of mercury is placed in the system to fill the cell chambers 27, wells 34 and 37, and decomposer 43 to normal levels, and the pump is set in operation. Mercury then begins to shower through the inlet piece 47, enter the chamber 23 of the top frame 11 through the trap at 36 and the slit 25, and ow gradually down the cathode plate 16. Overllowing the well 37, it showers through the outlet 40 and proceeds in sequence through the chambers of the frames 11a and 11b, and then into the decomposer 43. At the same time saturated salt brine is supplied to all chambers to till them to the level of the overflow holes 28. The mercury and brine ow in parallel through each chamber. The various chambers are in parallel with respect to brine circulation, but in series for the mercury ow.

Electrical potential is supplied across the bus-bars 29 and 31, with the top bar 29 anodic. Current then ilows vertically downward through the cell series, viz. through the connectors 30, anode rods 18, anodes 19, the brine in the chamber 23, mercury layer, cathode 16, pipe caps 17 to the next set of anode rods 18 and thence in series through the lower unit cells of the series.

In each chamber, the electrolysis of sodium chloride liberates sodium at the flowing mercury cathode where it alloys with the mercury to produce a weak amalgam. As the mercury proceeds in series through the cells, its sodium content increases. The flow rate of the mercury is controlled so that the concentration of the amalgam leaving the lowest cell for the decomposer is at a conventional value, e. g. about 0.03 to 0.3 per cent by weight of sodium. In the decomposer, this amalgam reacts with fresh water to produce sodium hydroxide solution and hydrogen, which are withdrawn together through the outlet 49 and then separated. The rate of water flow to the decomposer may be controlled so that sodium hydroxide solution of as high as per cent Strength is produced. The entering amalgam is virtually denuded of sodium, so that essentially pure mercury leaves through the outlet 44.

During the electrolysis, chlorine liberated at the anodes 19 rises above the electrolyte level in each chamber 23 and fills the free space. It escapes continuously with the spent brine through the outlets 28 and is recovered.

From the foregoing, it will be appreciated that the cell series of the invention eliminates all external connections between unit cells, and hence exhibits the high power efciency characteristic of bipolar electrode cell At the same time, the series is extremely cornpact, permitting high production rates per unit iloor space and also avoiding high power loss by heat radiation. The design is simple, rugged, comparatively inexpensive, and permits great flexibility in operation.

It is realized that others have described mercurycathode electrolytic cell series in which the unit cells are vertically superposed, and no claim is made to such structure broadly.

Attention is directed to a divisional application, Serial No. 466,038, tiled November l, 1954, which relates to 5 the details of the current interrupter in the mercury path between unit cells.

What is claimed is:

A cell series for the mercury-cathode electrolysis of alkali-metal halide brines comprising: a plurality of vertically superposed frames, each having a bottom partition therein, dening superposed electrolysis troughs; an elongated cathode member in each frame supported on the bottom partition thereof; anode members secured in each frame beneath the partition and connected electrically to the cathode above them in the same frame, such anodes being disposed a spaced distance above the cathode in the next lower frame; a cover for the topmost frame supporting anodes a spaced distance above the cathode in such frame; electrical terminals connected respectively to the anodes in the cover and to the cathode of the lowermost frame; adjustable spacing means holding each frame so that the cathode therein is slightly inclined from the horizontal and at a slope opposite to that of the cathode of the next higher frame; an inlet and an outlet well for mercury within each frame at opposite ends thereof, the inlet well being at the higher end, each such outlet well having an opening in the bottom thereof in vertical registry with the inlet well of the next adjacent frame; guide means within each such opening for conveying mercury from the outlet to the next lower inlet; a mercury trap at each end of each frame between the well at such end and the electrolysis trough in the frame, such trap permitting free mercury communication between the well and the trough but excluding communication of other fluids therebetween; means for circulating mercury to the uppermost frame inlet and withdrawing it from the lowermost outlet; individual devices within the guide means at each registering outlet-inlet well opening and also at the uppermost frame inlet and the lowermost frame outlet for breaking the electrical continuity of the mercury ow at each such region; sealing means between each pair of adjacent frames and between the topmost frame and the cover, along the entire periphery of the electrolysis trough in each frame, such means rendering the superposed troughs into a series of closed electrolysis chambers; and passages for circulating brine through each chamber and for collecting gaseous products therefrom.

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